Parabolic trough solar collectors systems generally comprise a horizontally extending reflector having a parabolic cross section, the reflector directing radiation from the sun to a horizontal focus line where a tubular conduit known as a collector tube is located. The collector tube is filled with a heat transfer fluid which is heated by the reflected solar radiation. The parabolic surface acts as a concentrator for directing an optimum amount of energy to the collector tube. Such parabolic trough solar collector systems are currently being utilized with large scale electrical generation plants to concentrate sunlight onto collector tubes placed at the focus line of the parabolic trough. Existing systems use synthetic oil as a thermal transfer fluid circulating in the collector tubes. The oil is heated to approximately 400 degrees C. and then sent to heat exchangers to produce superheated steam. The superheated steam is then sent to a standard steam turbine generator where it is converted into electricity.
The cost to generate electricity using the existing parabolic trough systems remains much higher than the cost to produce electricity using existing conventional energy sources such as coal, natural gas and nuclear. These higher costs are partly due to the complexity of the existing parabolic trough support structure. These expensive structures include the trusses that support the parabolic reflector. These trusses are attached to a mechanical pivot which allows the assembly to slowly and smoothly follow the arc of the sun. The pivot is then attached to a foundation. Each component of this existing assembly needs to support the weight of the parabolic trough assembly while resisting structural and wind loadings. Existing parabolic troughs in operation today are limited in size due to the cost/benefit relationship of structure to the width of the parabolic reflector. The wider the trough, the more structure is required to support the reflective parabolic surface and to resist wind loads as well as requiring a larger pivot mechanism.
The weight of the collector tube connected to the trough adds to these structural loadings. Likewise having the collector tube attached to the parabolic trough makes it necessary for the collector tube to incorporate flexible and/or rotational joints to allow the tube to move as the system follows the focus line of the parabolic trough through the day's arc. Existing parabolic troughs require multiple joints to allow the collector tube to remain in the focus line of the parabolic reflector. These joints limit the operating temperatures and pressures of the collector tube, and provide a source for loss of thermodynamic efficiency, wear and possible leakage.
Existing parabolic trough systems used primarily for electric generation use synthetic oil as the thermal collection fluid running inside the collector tube. The oil is used partly due to the many flexible joints needed in the collector tube system. Synthetic oil is used in existing systems partly because it does not impose the high pressures that would be encountered should there be an attempt at producing superheated steam directly in the collector tube. The oil in existing systems is then sent to a heat transfer station where the thermal energy of the oil is converted into superheated steam. This heat transfer results in some loss of energy efficiency. Existing parabolic trough systems using synthetic oil are further limited by the maximum operating temperature of existing synthetic oil which currently is about 400 degrees C. Above 400 degrees C., existing synthetic oil's thermal stability degrades leading to lower heat transfer efficiency and an increased creation of degradation by-products. The 400 degree C. operating temperature limitation of the synthetic oil also represents the operating temperature limitation of the steam being sent to the turbines for the production of electricity. The optimum operation temperature for an electrical generating steam turbine is much higher than 400 degrees C. This operating temperature limitation imposed by the synthetic oil further reduces the efficiency of existing systems using synthetic oil. The combination of the width limitation on the parabolic trough and the operating limitations on the collector tube results in lower overall efficiencies and higher costs for existing parabolic trough solar collector systems.